so, which one is correct? why?
"I haven't met..." is correct. "Haven't" is past tense, so the corresponding verb must also be past tense, in this case "met".

"I haven't met you before."
"I haven't been to London."
"I haven't climbed Mount Everest."
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is HASN'T also a past tense?
Oops... sorry. I have to correct myself. "Have" is not past tense. However, when we are saying "have not" it can relate to a past tense situation. It is a present tense, referring to a present condition of something not experienced in in the past, for example:

"I have not eaten today."

The same goes for using "has" (has not / hasn't), which refers to third person (he/she/it):

"He has not eaten today."

See this Wiktionary Simple English link for the word "have": http://simple.wiktionary.org/wiki/have
Yayya Lautner so, which one is correct? why?
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